This invention relates to a process and apparatus for sealing semiconductor packages made from a thermoplastic resin.
As resins for sealing semiconductor elements such as diodes, transistors, etc., or integrated circuits of these semiconductors (hereinafter referred to as a "semiconductive material"), there have been used epoxy series and silicone series thermosetting resins mainly. As molding methods for sealing with these resins, there have been used so-called transfer molding, casting, dipping and potting methods individually depending on objects for sealing. Among them, transfer molding using an epoxy resin is a typical method.
Plastic packages obtained by using plastics such as epoxy resins, etc., have a remarkable feature in that they can be produced in a lower cost due to collective seal molding of a number of lead frames mounting semiconductive materials compared with ceramic or metal packages by so-called hermetic seal using ceramics and metals. But since the thermosetting resins are used, there is a great defect in the molding steps. That is, it is necessary for several hours to carry out a postcure step in order to completely cure the thermosetting resin used for package after the seal molding. Therefore, it is difficult to combine smoothly intervals among individual steps and to automate the whole process comprising a dividing step of dividing semiconductor element built-in wafers into chip units, and a construction step including a die bonding step for fixing the chips on mounting portions of lead frames, a wire bonding step for connecting electrodes of chips to leads, a resin sealing step, a lead post-working step, and a marking step. In such a process, there is used a transfer molding apparatus having a structure in that a lead frame as a center axis is positioned horizontally for handling and transferring.
In order to solve the defects of the transfer molding mentioned above, Japanese patent unexamined publication No. 181129/82 proposes an apparatus for sealing semiconductor packages as shown in FIG. 4 wherein a lead frame positioned horizontally is taken as a center axis for handling and transferring as usual in the semiconductor industry, two flat moldings made from a thermoplastic resin previously molded are positioned at symmetrical places above and below, sealing surfaces of the flat moldings are melted by heating with hot-air heaters positioned symmetrically above and below, and a lead frame mounting a semiconductive material is sandwiched between the flat moldings and joined integrally under pressure. In FIG. 4, mold parts 3 and 3' for sealing individually have a cavity 5, 5' for receiving a flat molding, a cavity for preheating a sealing surface of flat molding, a cavity (not shown in the drawing) for heating the sealing surface of flat molding, and a cavity for integrally joining with a lead frame 1, and rotate around rotating shafts 8 and 8'. In FIG. 4, numerals 6 and 6' are hot-air heaters and 9 and 9' are hot-air preheaters.
This apparatus shows remarkably good sealing properties in sealing operation in a short time, but produces a fatal defect mentioned below in a long period sealing operation due to convection of hot-air since the hot-air heaters 6 and 6' and the hot-air preheaters 9 and 9' are positioned above and below. That is, the hot-air heater 6, the hot-air preheater 9 and the mold part 3 for sealing positioned at an above place show a tendency to be overheated by the convection phenomenon of hot-air. Further, since the upper and lower hot-air preheaters 9 and 9' have hot-air blow directions opposite to each other via the lead frame 1, it becomes difficult to control the heating conditions necessary for maintaining the sealing properties, i.e., the viscosity of molten resin of sealing surfaces of flat moldings, in a predetermined range. As a result, the sealing properties are damaged; this is a fatal defect. Further, considering the maintenance of the apparatus, since various parts of the apparatus are symmetrically positioned above and below of the center axis which is the lead frame 1 positioned in a horizontal attitude, the arrangement of parts particularly in the lower portion becomes very difficult, which results in constituting a hindrance to smooth maintenance and control of the apparatus.